The present invention relates generally to a light source optical system, and more particularly to a light guide illumination apparatus for endoscopes.
As already well known in the art, the color imaging mode for electronic scopes generally includes the “mosaic filter mode” and the “field sequential illumination mode”. In the mosaic filter mode, a solid-state image pickup device with a color filter formed per pixel is used on an imaging plane to obtain color images by white illumination. In the field sequential mode, on the other hand, a black-and-white solid-state image pickup device is used, and color filters for R, G, B, etc. are mounted in a light source apparatus that is an illumination means to time-divide illumination light thereby obtaining color images.
The former allocates colors to the limited number of pixels; so it is lower in resolving power than the latter using a monochromatic solid-state image pickup device, and renders it difficult to control color reproduction because the color filters are on the imaging plane of the image pickup device. The latter enables color reproduction to be controlled by altering the characteristics of the color filters mounted in the light source apparatus, and makes better color reproduction possible.
For the color filters located in the light source apparatus operating in the field sequential mode, the interference type is generally used. With this interference type filters, there is a change in the transmittance (reflectance) characteristics depending on the angle of incidence of light rays. For instance, when it comes to a filter designed such that the desired characteristics are obtained at the angle of incidence of 0°, the larger the angle of incidence, the more the transmitted wavelength shifts or ripples to a short wavelength side and the more of deviation there is from the desired characteristics with the result that color reproduction goes worse. To obtain the desired characteristics with the interference type filters yet with high precision, it is necessary to keep the angle of incidence from growing wide: this requirement is an imperative challenge to designing light source optical systems.
A typical light source optical system of the field sequential illumination mode with color filters located in it is set forth in Patent Publication 1. In this light source optical system, as shown in FIG. 8, light emanating from a light source 1 is somewhat moderately collected by a front unit 2a of a collective lens system 2, entering a filter 5. The light is then collected by a rear unit 2b of the collective lens system 2, entering the entrance end surface of a light guide 6.
Such arrangement has some defects: it renders it difficult to diminish the diameter of light beams on the color filter surface, resulting in a bulky filter size and thus an increase in the size of the light source optical system. In addition, the angle of incidence of light onto the filters is wide with the result that color reproduction becomes inferior due to the above change in the angle-of-incidence characteristics.
On the other hand, a typical optical system of the field sequential illumination mode—which takes care of the angle of incidence of light rays onto the interference type color filters—is set forth in Patent Publication 2. As shown in FIG. 9(A), this optical system is built up of a collective optical system 2 for collecting parallel light beams emanating from a light source 1, a magnification conversion optical system 3 for reducing the diameter of light beams after passing through the collective optical system 2, and a positive lens group 4 that is located on an entrance end surface side of a light guide 6 with respect to the magnification conversion optical system 3 and collects light from the magnification conversion optical system 3 onto the entrance end surface of the light guide 6, with a color filter 5 interposed between the magnification conversion optical system 3 and the positive lens group 4. With the optical system of such arrangement, the parallel light beams from the light source 1 are subjected to magnification conversion into reduced, substantially parallel light beams. It is thus possible not only to diminish the diameter of light beams onto an interference coating surface thereby reducing apparatus size but also to keep the angle of incidence from growing wide thereby achieving satisfactory color reproduction. When a monochromatic, full-frame transfer type CCD is employed, use is often made of a color wheel 10 comprising R, G and B color filters 11R, 11G and 11B and a light block plate 12 for controlling exposure and light blocking. With this color wheel it is possible to improve brightness more because the smaller the diameter of light beams at that wheel position, the higher the efficiency of light use becomes.
In recent years, in addition to general viewing in three colors: RGB, there have been new viewing methods developed as represented by the AFI method for viewing the in vivo self-fluorescence, the NBI method for irradiating blood vessels with narrow band light to view blood vessel images in high contrasts, etc., and their effectiveness has been reported as well. With new such viewing methods, images are obtained by irradiation with the desired illumination light in combinations of color filters. If various viewing methods can be implemented with a single light source apparatus, it is unnecessary to get one dedicated light source apparatus ready for each viewing method, offering greater user merits.
To be compatible with various viewing methods using a single light source apparatus, color filters dedicated to each viewing method must be mounted in an associated light source optical system: as color filter space grows wider, it allows for more color filters well fit for many viewing methods.
For the optical system of FIG. 9(A), it is preferable to make sure a longer distance between the magnification conversion optical system 3 with the color filter 5 mounted in it and the positive lens group 4. However, making this distance much longer offers a problem that the quantity of incident light decreases, because, as shown in FIG. 9(B), off-axis light rays at the positive lens group 4 gains some height, rendering it difficult to increase the aperture of the positive lens group 4, and to correct aberrations for the purpose of making light incident onto an optical fiber forming the light guide 6.    Patent Publication 1: JP(A) 7-303604    Patent Publication 2: Patent No. 2826315